Ever since the beginning of the space age, communication satellites have been an important application of space technology. The first communication satellite was Telstar. At the time, it was an extraordinary technological achievement. It was designed, built and operated by Bell Telephone Laboratories, Holmdel, N.J., USA.
Communication satellites receive and transmit radio signals from and to the surface of the Earth. For Telstar, being the first and only communication satellite of its time, there was no issue of interference between its radio signals and radio signals associated with other communication satellites. But this situation changed quickly as the technology advanced and demand for communication-satellite services exploded.
At the present time, demand for all manners of wireless communications has reached new highs, and the radio spectrum has become a very valuable commodity. In January, 2015, as part of a government auction, fifty MHz of radio spectrum have fetched an unprecedented $39.5 billion. Therefore, being able to establish a radio communication system that does not require dedicated spectrum would be a significant advantage.
When it comes to communication satellites, the so-called geostationary satellites are a well established type of satellites that have provided a variety of useful services for many decades. Geostationary satellites orbit the Earth in a plane that intersects the Earth at the Equator (the “equatorial plane”). They orbit at a distance from the Earth such that the period of their orbit is exactly one sidereal day. The geostationary orbit is a circle and, therefore, geostationary satellites go around the Earth at exactly the same rate as the Earth rotates around its axis. As a result, each geostationary satellite appears, from the Earth, at a fixed position in the sky, as if it were mounted on an extremely tall tower.
That virtual tower is, indeed, very high: about 36,000 km, or almost six times the Earth's radius. The great distance between geostationary (commonly abbreviated as “GEO”) satellites and the surface of the Earth has several undesirable consequences, including a need for higher transmitted signal power, difficulty in generating small transmission footprints, and an annoying delay in communications, among others. For certain applications, including internet services, satellites that orbit closer to the surface of the Earth might be better suited.
FIG. 1 depicts a satellite in a low earth orbit (commonly abbreviated as “LEO”). While there is no international standard for the exact meaning of orbit-type designators such as “GEO”, “LEO”, and “MEO” (which stands for “middle earth orbit”) they are commonly used in the art. LEO orbits are generally assumed to be orbits where the satellite orbits no more than about 2000 km above the surface of the Earth. In FIG. 1, the LEO orbit is depicted by a dashed circular line as LEO polar orbit 150. Because the orbit is circular, the satellite orbits the Earth 110 at an approximately constant distance from the Earth's surface. In FIG. 1, that distance is small compared to the Earth's radius; at the scale of the Figure, it corresponds to approximately 900 km.
The type of LEO orbit depicted in FIG. 1 is known as a “polar” orbit because it passes over the North pole and the South pole. One advantage of polar orbits is that the satellite passes over all latitudes. As the Earth rotates (while the plane of the orbit remains approximately unchanged) the satellite will pass over different areas of the Earth. With an appropriately chosen orbital period, the satellite can be made to pass over every place on Earth, after a large-enough number of orbits. For this reason, polar orbits (or nearly-polar orbits) are often chosen for earth-surveying satellites.
FIG. 2 depicts the same LEO satellite and satellite orbit of FIG. 1, together with a depiction of a geostationary satellite for the purpose of emphasizing the different orbital sizes, which are drawn to scale. In the figure, GEO satellite 230 orbits the Earth 110 in the plane of the Earth's Equator 210. The plane is depicted by a dashed line as equatorial plane 220. As predicted by geometry, LEO polar orbit 150 intersects the equatorial plane at right angles. There are two points of intersection. In the art, the two points of intersection between an orbit and equatorial plane 220 are known as “nodes”. The North pole is on one side of equatorial plane 220, and the South pole is on the other side. The satellite travels along its orbit in the direction shown in FIG. 2 as “101 direction of motion”. At one of the nodes, it passes through equatorial plane 220 going toward the North pole. That node is commonly referred to as the “ascending” node, while the other node is commonly referred to as the “descending” node.
The nomenclature of “ascending” and “descending” nodes can also be defined when satellite 230 is not a geostationary satellite and its orbit is not in the equatorial plane. For example, satellite 230 might be a geosychronous satellite orbiting the Earth in the so-called GEO stable plane which is inclined, relative to the equatorial plan, at an angle of about 7.3°. For any plane that passes through the center of the Earth, the North pole lies on one side of the plane, and the South pole lies on the other side. As the satellite travels along its orbit, at one of the nodes it passes through the plane going toward the side of the plane where the North pole lies. That node is referred to as the “ascending” node, while the other node is referred to as the “descending” node,
FIG. 3 presents a more detailed depiction of the LEO satellite and its relationship to the surface of the Earth below it. In particular, it shows the case where the LEO satellite is a communication satellite. (In this detailed figure and in many of the subsequent figures, continental outlines on the surface of the Earth have been omitted to avoid clutter). The LEO satellite 140 is equipped with one or more radio antennas. The antennas transmit one or more radio signals toward the surface of the Earth 110. Such transmissions are shown in the figure as radio transmissions 310. The radio transmissions can be received by receivers located on the surface of the Earth within a certain coverage area depicted as coverage area 320. Outside of coverage area 320, the radio signals from the satellite are expected to be too weak for adequate reception; indeed, the radio antennas on the satellite can be deliberately designed to make such radio signals weak for the purpose of limiting interference caused by those signals to other receivers outside coverage area 320.
FIG. 4 shows what happens when GEO satellite 230 is a communication satellite that serves a portion of the Earth that overlaps coverage area 320 (not explicitly shown in this figure). Like the LEO satellite, the GEO satellite is also equipped with one or more radio antennas that transmit one or more radio signals toward the surface of the Earth. Such transmissions are shown in the figure as radio transmissions 410. The radio transmissions are aimed at a portion of the Earth depicted in the figure as coverage area 420.
LEO satellite 140 is shown FIG. 4 as being very close to coverage area 420; therefore, even though coverage area 320 is not shown explicitly, coverage area 320 clearly overlaps coverage area 420, at least in part. If radio transmissions 410 and radio transmissions 310 comprise radio signals in the same part of the radio spectrum, there is the potential for interference between transmissions from the LEO satellite and transmissions for the GEO satellite.
Under International Telecommunications Union (ITU) rules, non-geostationary (NGSO) satellites such as LEO satellite 140 are allowed to use the same spectrum as GEO satellites under certain conditions. In particular, NGSO satellites must not interfere with GEO satellites that use the same spectrum frequencies. The ITU rules set out specific guidelines as to how much radio signal power into a GEO satellite's terminal can be created without needing to “coordinate” or talk with and get approval from the GEO satellite's operator.
FIG. 5 depicts a possible scenario for how unacceptable interference can be caused by a LEO satellite to the signal from a GEO satellite. On the surface of the Earth 110, there is a GEO receiver 510 that is attempting to receive a radio signal 520 from a GEO satellite. However, GEO receiver 510 is located inside coverage area 320 where radio transmissions 310 from LEO satellite 140 comprise radio signals that might use frequencies that fall within the spectrum band used by the GEO satellite. To make matters worse, LEO satellite 140 lies along the line of sight between GEO receiver 510 and the GEO satellite. Therefore, the path followed by GEO radio signal 520 on its way to GEO receiver 510 passes near LEO satellite 140, and, from the point of view of GEO receiver 510, both the desired radio signal 520 and the interfering radio transmissions 310 arrive from the same direction. Under these conditions, without employing further mitigation techniques, and depending upon the power spectral density of the interfering radio transmissions 310, the GEO receiver 510 may have difficulty achieving good reception of radio signal 520.
FIG. 6 depicts a technique commonly used to mitigate the type of interference situation depicted in the previous figure. In FIG. 6, LEO satellite 140 simply reduces the size of the coverage area 620 over which it provides communication services. With such a reduction, GEO receiver 510 is now located outside coverage area 620. It is still true that, from the point of view of GEO receiver 510, the desired radio signal 520 arrives from the same direction as any interfering radio transmissions from LEO satellite 140 that “spill over” outside the boundary of reduced coverage area 620. However, as already mentioned, the radio antennas on LEO satellite 140 can be designed such that radio signals that spill over outside of coverage area 620 are weak. Antennas can be designed such that those signals are as weak as necessary to meet ITU limits.
What about receivers that are inside reduced coverage area 620? FIG. 6 depicts one such receiver as GEO receiver 515. It is located near the boundary of reduced coverage area 620, and it is attempting to receive radio signal 525 from the GEO satellite. From the point of view of GEO receiver 515, the desired radio signal 525 does not arrive from the same direction as interfering radio transmissions from LEO satellite 140. There is a non-zero angle between the two directions of arrival. Because of this angle, GEO receiver 515 is better able to discriminate between radio signal 525 and radio transmissions 140. In other words, thanks to the angle, the power spectral density of radio transmissions 140, as received by GEO receiver 515, can more easily meet the ITU limits. Of course, the larger the angle, the greater the benefit; therefore, the worst-case position for a GEO receiver, within reduced coverage area 620, is where GEO receiver 515 is depicted in FIG. 6, near the northern boundary of reduced coverage area 620, where the angle is smallest. Such smallest angle is depicted in FIG. 6 as angular separation 630. Because radio signal 520 is substantially parallel to radio signal 525, angular separation 630 can be measured relative to either of the two radio signals, as shown in the figure.
The presence of angular separation 630 makes it possible for GEO receiver 515 to achieve good reception of desired radio signal 525 even in the presence of interfering radio transmissions 610 from LEO satellite 140. This is true as long as angular separation 630 is sufficiently large. How large it needs to be depends heavily on the characteristics of the antenna used by GEO receiver 515 for receiving the radio signal 525. As it turns out, Earth-based receivers of GEO satellite signals must typically use so-called high-gain antennas. Such antennas exhibit excellent angular selectivity which enables them to reject interfering signals with an angular separation as small as a few degrees. Moreover, there are well-defined standards for the characteristics of GEO receiver antennas. As a result, a LEO satellite can implement a carefully chosen angular separation and be confident of not interfering with GEO receivers.
The technique illustrated in FIG. 6 can be easily implemented if radio transmissions from the satellite are in the form of multiple independent beams aimed in different directions. In that case, the coverage area can be reduced simply by turning off some of the beams. However, there are several disadvantages. One disadvantage is that, of course, by reducing the coverage area, the effectiveness of a satellite is reduced. It can serve fewer communication terminals on the surface of the Earth, and the volume of traffic that the satellite carries is reduced. But another important disadvantage is that the technique is of limited utility. The situation depicted in FIG. 6 is applicable to areas of the Earth at mid to high latitudes. In the depiction of FIG. 6, North is up (same as in the preceding figures) and the angle of the surface of the Earth corresponds to a latitude of 47°. Most of the population of the Earth lives at or below this latitude. Yet, with the geometry of FIG. 6, it is clear that the coverage area served by LEO satellite 140 must be severely curtailed, if interference to GEO receivers is to be avoided. The situation becomes much worse at lower latitudes, as is illustrated in the next figure.
FIG. 7 shows what happens when the interference mitigation technique of FIG. 6 is attempted at lower latitudes. In particular, FIG. 7 depicts LEO satellite 140 when its orbit 150 brings it near the Equator. The figure also depicts GEO receiver 710, which is attempting to receive radio signal 720 from a GEO satellite. It is clear that the direction of arrival of radio signal 720, as received at GEO receiver 710, is the same as the direction of arrival of interfering radio transmissions 712 from the LEO satellite. The geometry is such that this is true even in the middle of coverage area 722. No extent of reduction in the size of coverage area 722 will achieve a reasonable angular separation between radio signal 720 and the interfering radio transmissions 712. The only way for LEO satellite 140 to meet the ITU rules is to cease all transmissions, or to potentially operate at a significantly lower power spectral density, thereby negatively impacting service provided by satellite 140.
Since there are plural orbiting satellites, this problem always exists for one of more of them at any given time. It is, of course, undesirable to have to turn off a satellite, especially if this has to happen consistently, always at the same location. Such location will not receive communication services. In this case, the problem occurs consistently at all low latitudes. FIG. 7 illustrates the difficulty of providing communication services to areas of the Earth at low latitudes with LEO satellites, if those satellites must share the radio spectrum with GEO satellites.
In FIGS. 3-7, radio transmissions from a satellite are simply depicted as a cone emanating from the satellite. In typical communication satellites, such transmissions comprise multiple independent beams, each carrying one or more radio signals. Multiple beams can be generated via, for example, multiple independent antennas, or via a single antenna reflector with multiple feeds, or via antenna arrays, or via other means. FIGS. 8 and 9 illustrate the use of multiple beams with LEO communication satellites.
FIG. 8 depicts a LEO communication satellite 840 capable of transmitting multiple independent beams 810. The beams are aimed at the surface of the Earth. Each beam provides radio coverage to a portion of the overall coverage area. Ideally, different beams should not overlap, but, of course, a certain amount of overlap is unavoidable and, indeed, necessary to avoid coverage gaps between beams. Nonetheless, it is customary to depict the pattern of coverage as if the beams were disjoint.
FIG. 9 depicts an example of a beam-coverage pattern. The nineteen hexagons represent the footprints of nineteen beams emanating from the satellite. The footprints lie on the surface of the Earth. All the hexagons are of equal size, such that all the beams experience approximately equal traffic loads. Note that the hexagons near the periphery (hexagons 8 through 19) result from beams that reach the ground at a low elevation angle, while the hexagons near the center (such as hexagon 1) result from beams that reach the ground from a near-vertical direction. The actual pattern of beams emanating from the satellite, and the antennas that generate the beams, must be adjusted such that the beams near the periphery are of a different shape, compared to the beams near the center, such that the footprints on the ground achieve the desired regular pattern. It is well known in the art how to design an antenna system that generates a three-dimensional pattern of beams such that, when the beams reach the ground, they form footprints in a desired coverage pattern.
A LEO satellite in a circular orbit travels around the Earth at a substantially constant distance form the surface of the Earth. The satellite is equipped with an attitude control system for controlling the orientation of the satellite. The orientation is adjusted such that the satellite antennas are always pointed toward the surface of the Earth, and such that the geometry of the antennas relative to the surface of the Earth below the satellite is unchanged as the satellite travels along its orbit. This is done to ensure that the coverage pattern shown in FIG. 9 remains unchanged under the satellite and travels together with the satellite, as the satellite travels along its orbit.
While in orbit, communication satellites must maintain their antennas in a precise orientation, relative to the Earth. The antennas, then, can transmit radio signals aimed at the surface of the Earth in a geometric pattern devised to provide good coverage for radio terminals on the surface of the Earth. For LEO satellites, that pattern usually comprises a plurality of independent beams, each of which covers a portion of the overall coverage area. FIGS. 8 and 9 illustrate such multibeam coverage.
In the prior art, LEO communication satellites follow circular orbits and maintain a fixed orientation, relative to their direction of motion, with the beam pattern aimed downward, toward the surface of the Earth. This way, the coverage pattern on the surface of the Earth moves along with the satellite without changing its shape.
Such a coverage pattern achieves good performance, if the satellite can use dedicated spectrum. But, if the satellite must share spectrum with one or more GEO satellites, the problems illustrated in FIGS. 6 and 7 require that a number of beams be turned off over large portions of the orbit, and all the beams must be turned off when the satellite is near the equatorial plane.